This invention pertains generally to the field of electric motors and to power conversion systems for driving electric motors.
Three-phase induction machines are presently a standard for industrial electrical drives. Relatively low cost, high reliability, robustness and maintenance-free operation are among the reasons three-phase induction machines are replacing DC drive systems. The development of power electronics and signal processing systems has eliminated what had been one of the greatest disadvantages of such AC systems, that is, the issue of control. With modern techniques of field oriented vector control, variable speed control of induction machines is readily achievable.
The need to increase system performance, particularly when placing limits on the power ratings of power supplies and semiconductors, motivates the use of a phase number other than three, and increases the need for new pulse width modulation (PWM) techniques, new machine design criteria, and the use of harmonic current and flux components.
In a multi-phase system, here assumed to be a system that comprises more than the conventional three phases, the machine output power can be divided into two or more solid state inverters that can each be kept within prescribed power limits. In addition, having additional phases means that additional degrees of freedom are available for further improvements in the drive system.
With split-phase induction machines, and an appropriate drive system, the sixth harmonic torque pulsation, typical in a six-step three-phase drive, can be eliminated. See, A. R. Bakhshai, et. al., xe2x80x9cSpace Vector PWM control of a split-phase induction machine using the vector classification technique,xe2x80x9d Proc. of Applied Power Electronics Conf. and Exposition, 1998, Vol. 2, February 1998, pp. 802-808; R. H. Nelson, et al., xe2x80x9cInduction machine analysis for arbitrary displacement between multiple windings,xe2x80x9d IEEE Trans. on Power Apparatus and Systems, Vol. 93, May 1974, pp. 841-848. In addition, air gap flux created by fifth and seventh harmonic currents in a high power six-step converter-fed system is dramatically reduced, with a penalty of increased converter harmonic currents. See, L. U. Xu et al., xe2x80x9cAnalysis of a novel stator winding structure minimizing harmonic current and torque ripple for dual six-step converter-fed high power AC machines,xe2x80x9d IEEE Trans. on Industry Applications, Vol. 31, January 1995, pp. 84-90. PWM techniques are employed to overcome this problem by eliminating the harmonic current in the modulation process when the power ratings are not prohibitive.
Dual-stator machines are similar to split-phase machines, with the difference that the stator groups are not necessarily equal. A dual-stator machine with different numbers of poles in each three-phase group has been developed to obtain controllability at low speeds. A. R. Munoz, et al., xe2x80x9cDual-stator winding induction machine drive,xe2x80x9d IEEE Trans. on Industry Applications, Vol. 36, September 2000, pp. 1369-1379, and U.S. Pat. No. 6,242,884, entitled Dual Stator Winding Induction Machine Drive. Two independent stator windings have also been used for an induction generator system, as described in O. Ojo, et al., xe2x80x9cPWM-VSI inverter-assisted stand-alone dual-stator winding induction generator,xe2x80x9d IEEE Trans. on Industry Applications, Vol. 36, November 2000, pp. 1604-1611. In this system, one set of windings is responsible for the electromechanical power conversion while the second set is used for excitation purposes. A PWM converter is connected to the excitation windings and the load is connected directly to the power windings.
A six-phase machine, a particular case of a split-phase or dual-stator machine, can be built by splitting a three-phase winding into two groups. Usually these three-phase groups are displaced by thirty electrical degrees from each other. This arrangement provides an asymmetrical six-phase machine because the angular distance between phases is not all the same. See, G. Oriti, et al., xe2x80x9cAn inverter/motor drive with common mode voltage elimination,xe2x80x9d Thirty-Second IAS Annual Meeting, IAS"" 1997, Vol. 1, October 1997, pp. 587-592. The analysis of an induction machine for multiple phases and arbitrary displacement between them is presented in R. H. Nelson, et al., supra, where a six-phase induction machine is used as an example and an equivalent circuit has been derived. The dqO model for a six-phase machine is described in T. A. Lipo, xe2x80x9cA space d-q model for six-phase induction machines,xe2x80x9d Proc. Of the International Conference on Electrical Machines, Athens, September 1980, pp. 860-867.
Reliability is one of the advantages of a six-phase system. If one of the phases failsxe2x80x94either in the machine or in the power converterxe2x80x94the system can still operate on a lower power rating since each three-phase group can be made independent of the other. If one phase is lost, the six-phase machine can continue to be operated as a five-phase machine, for example, as described in J. Fu, et. al., xe2x80x9cDisturbance free operation of a multi-phase current regulated motor drive with an open phase,xe2x80x9d IEEE Trans. on Industry Applications, Vol. 30 September 1994, pp. 1267-1274.
The inherent third harmonic component in the winding functions of such machines indicates the possibility of using third harmonic currents to improve performance. H. A. Toliyat, et al., xe2x80x9cAnalysis of a concentrated winding induction machine for adjustable speed drive applications. I. Motor Analysis,xe2x80x9d IEEE Trans. on Energy Conversion, Vol. 6, December 1991, pp. 679-683; H. A. Toliyat, et al., xe2x80x9cAnalysis of a concentrated winding induction machine for adjustable speed drive applications. II. Motor design and performance,xe2x80x9d IEEE Trans. on Energy Conversion, Vol. 6, December 1991, pp. 684-692. Torque improvement can be obtained by using multi-phase windings with injection of third harmonic currents. A nine-phase induction machine with third harmonic injection has been investigated. S. S. P. Liou, xe2x80x9cTheoretical and experimental study of poly-phase induction motors with added third harmonic excitation,xe2x80x9d Masters Thesis, University of Texas at Austin, 1985. The complexity of the power system, which includes series and parallel transformers, increases the system costs and requires evaluation for particular applications. The use of a voltage-controlled system does not guarantee the phase alignment between fundamental and third harmonic currents, especially at low speeds, and a poor low speed and dynamic behavior can be expected for such a system.
In the present invention, a significant increase in torque may be provided from an induction motor in the motor drive system of the invention as compared to the torque provided by a conventional induction motor of the same physical dimensions and characteristics. Potential increases in torque output up to 40% may be obtained from a modified induction motor in accordance with the invention, utilizing conventional commercially available stator core and rotor constructions, with a modification of the winding of the electrical conductors on the stator. The modified induction motor is readily constructed in accordance with commercial motor manufacturing processes and requires minimal additional materials.
A preferred motor drive system in accordance with the invention includes a motor having a stator with a core and two three-phase windings wound on the core, with the two windings separated spatially by 30 electrical degrees. Each of the two windings has three terminals and a neutral terminal by which power may be applied to the windings. An induction machine rotor, such as a squirrel cage rotor, is mounted for rotation within the stator. A first power supply is connected to the terminals of a first of the stator windings and a second power supply is connected to the terminals of a second of the stator windings. The power supplies provide current to the two three-phase stator windings with the same fundamental frequency and with an additional component at the third harmonic of the fundamental frequencies. One of the power supplies provides current to one of the windings that is shifted in time by 30xc2x0 (of the fundamental) with respect to the current provided by the other power supply. Shifting one winding with respect to the other spatially by 30 electrical degrees and providing current at the fundamental frequency to the two windings separated in time by 30xc2x0 effectively allows the machine to operate with the same flux density distribution as a conventional three-phase machine having a single three-phase winding. The addition of the third harmonic component to the power supplied to the two windings effectively flattens the peak of the flux density distribution, allowing the fundamental component of the flux density to be increased until the composite air gap flux density reaches its prior peak level, thereby producing significantly enhanced torque. In addition, the third harmonic component itself contributes output torque. An increase of up to 40% in the torque production can be obtained as compared to a standard three-phase machine, for the same peak flux distribution. Alternatively, the tooth and core flux density may be constrained to fixed values, but the stator slot area and the amount of conductor wound in the slots increased to also allow a significant increase in torque production.
The power supplies for the motor drive system are preferably current controlled inverters, e.g., pulse width modulated current controlled inverters, which provide power to the two three-phase windings of the motor that is balanced in each of the two windings.
The present invention may also be implemented in a linear motor in which one or more stators with two three-phase windings are provided in a linear arrangement with a linear moving element (a xe2x80x9clinear rotorxe2x80x9d) mounted for translational movement adjacent to the stator(s).
The invention may be extended to machines having more than six phases. For example, the motor drive system may include a machine having nine phases, composed of three three-phase windings, with each set of windings provided with power from a three-phase power supply such as an inverter. In this case, the three three-phase windings are shifted with respect to each other by 20 electrical degrees. Third harmonic currents may be provided by the power supplies as a three-phase set of third harmonics which are balanced and sum to zero, in which case a neutral return line to the power supplies is not needed. The invention may also be utilized with wound rotor induction and synchronous motors.
Further objects, features and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings.